Silicon oxide powders, due to their large electrical capacity, are known to serve as superior negative electrode materials for lithium ion secondary batteries. And yet, the silicon oxide powders serving as the negative electrode materials for lithium ion secondary batteries are low in initial efficiency. A method known to address this problem is lithium doping (Li-doping). Specifically, Li-doping of the silicon oxide powder is intended to improve the initial efficiency by allowing initial charging to be done with the inhibition the formation of lithium compounds that do not contribute to the charging-discharging.
The Li-doping of the silicon oxide powder forms lithium silicate. Lithium silicate has a phase varying depending on an amount of the Li-doping. Specifically, with an increased amount of the Li-doping, lithium silicate has a phase varying from Li2Si2O5, through Li2SiO3, to Li4SiO4, as shown in chemical formulae below.SiO2+⅖Li→⅕Li2Si2O5+⅗SiSiO2+⅔Li→⅓Li2SiO3+⅔SiSiO+Li→¼Li2SiO4+¾Si  [Chem. 1]
Lithium silicate, at its amorphous state, is reactive with water, but through its crystallization, particularly in the case of Li2Si2O5, comes to be unreactive with water. This is favorable from viewpoints such as its handlability. It is therefore desirable that Li-containing silicon oxide powder having undergone the Li-doping have lithium silicate mostly in the form of a crystallized Li2Si2O5. Meanwhile, Li2SiO3 and Li4SiO4 in lithium silicate, even through crystallization, remain reactive with water, which is a reason for such problems as lithium elution during its handling process.
The crystallization process of lithium silicate in the Li-containing silicon oxide powder is accompanied by Si advancing toward crystallization. Crystallization of Si would have an adverse effect on cycle properties of batteries. While the amorphous Si in the absence of heat can retain its amorphous state by, for example, an electrochemical Li-doping, the electrochemical Li-doping however would permit the lithium silicate, too, to be amorphous, and would involve the difficulty in obtaining powders.
It is desirable in view of the above circumstances that Li-containing silicon oxide powders for use in negative electrode materials for lithium ion secondary batteries be the ones that have a crystallized lithium silicate, in particular, crystallized Li2Si2O5, and have an amorphous Si.
Such Li-containing silicon oxide powders are produced, for example, by a powder calcining-based method that includes mixing the silicon oxide powder with a powdered lithium source, and calcining the resultant mixture (Patent Literatures 1 to 4). The silicon oxide powders given here are produced by heating a mixture of silicon dioxide and silicon to form a silicon monoxide gas, cooling the resultant silicon monoxide gas for precipitation, and finely grinding the precipitate. The silicon oxide powders produced by such a precipitation method are mostly amorphous enough to have a lowered thermal expansion coefficient, and therefore are said to be advantageous from such viewpoints as accomplishing improved cycle properties.
However, such powder calcining-based Li-doping methods have a problem: lithium silicate formed by such methods includes not just water-insoluble Li2Si2O5 but also water-soluble Li2SiO3 and water-soluble Li4SiO4. This problem has been addressed by avoiding adding an increased Li-doping amount at the time of Li-doping. Nonetheless, the powders having undergone the Li-doping in a decreased amount represented by, for example, Li/O (atomic ratio)=0.2, which amount is considered from an equilibrium standpoint to contribute to the inhibition of the formation of Li2SiO3, in fact, exhibit peaks of Li2SiO3 in the observation by XRD measurement: the presence of this problem has been identified by experiments made by the inventors of the present invention.
Further decreasing the Li-doping amount would limit the formation of lithium silicate itself, depriving the Li-doping of its significance, which would make it impossible to hope for the enhancement of the initial efficiency intended by the Li-doping. Excessively increasing the Li-doping amount, in contrast, (for example, Li/O (atomic ratio)≥1), would cause the lithium silicate phase to be occupied by Li4SiO4 alone with Li2SiO3 absent, which is a reason for the exhibition of such an excessive activity as to make its usability inferior.
The inventors have found that the powder calcining-based Li-doping, because of involving heating, could involve disproportionation of silicon oxide that will produce the crystalline Si and that tends to occur at a lower temperature. Another tendency is that an increased doping amount in the Li-doping would lead to an increased amount of metal Si, as is clear from the chemical formulae set forth hereinabove.
In view of the above, the applicant's corporation has performed the powder calcining-based Li-doping at a decreased Li-doping amount and at a lower thermal treatment temperature. And yet, even at a heat treatment temperature at which the disproportionation of silicon oxide should intrinsically not take place (for example, not higher than 700° C.), the resultant powder undergoes the disproportionation of silicon oxide and, in observation by powder XRD measurement, exhibits peaks including a high peak attributed to the crystalline Si, which is a risk for reduced cycle properties.
To the knowledge of the applicant in view of the foregoing, Li-containing silicon oxide powders containing a crystallized lithium silicate that is mostly water-insoluble Li2Si2O5 and containing little crystalline Si are nonexistent.
Another approach, aside from the Li-doping, taken to enhance the cycle properties is allowing particles (powder particles) constituting silicon oxide to undergo a carbon coating treatment (C-coating). In Patent Literature 3, Li-doping is followed by C-coating; and in Patent Literature 4, C-coating is followed by Li-doping.